During my cinemicrographic studies on long-term growth of bone marrow cells, a new type of net-like stromal cell appeared in the culture. The cell was very large, measuring more than 500 micrometer in diameter. The cytoplasm was very thin, but enforced with cytoplasmic strands arranged like a fishing net. Frequent appearance and disappearance of small and large holes in the cytoplasm was observed. There were two nuclei, each containing a prominent nucleolus. The cell remained more or less stationary throughout a period of 44-hour continued cinemicrography and was still in good condition at the end of observation. These features suggested that this new type of cell might be the mysterious, not yet cultivated reticulum cell of the bone marrow. The cell was observed in a liquid culture system developed in this laboratory. Good growth of macrophages, granulocytes, and megakaryocytes was also observed. Short motion pictures of these cells are available.